A previous blog Ultrasonics - Degassing - What Gas? and Why? revealed why dissolved gas is detrimental to ultrasonic performance. This blog will describe various ways in which dissolved gas can be minimized and eliminated to facilitate ultrasonic cleaning. Since the most common liquids used in ultrasonic cleaning are water and water-based solutions, the following applies primarily to water although most liquids behave similarly. The temperature of a liquid has a significant impact on how much gas can be dissolved in that liquid. Cold water is able to dissolve much more gas than hot water. You may have noticed that when you put water in a pot on the stove and turn on the burner, small bubbles start forming in the water long before it actually boils. These bubbles are gas that is in excess of that the water can dissolve at the increased temperature provided by the burner. You will also note that these bubbles frequently form on the sides of the pot. This is because the contact of the water with the sides of the pot provide "nucleation sites" which facilitate the gas coming out of solution. Increasing the temperature of a liquid reduces the amount of gas that the liquid can dissolve. Ultrasonic cleaning tanks should be heated to the final operating temperature before use. The ideal preparation is to heat the liquid several degrees above the process temperature and then cool it to the process temperature before use. Doing this assures total degassing of the liquid at the process temperature. Increasing temperature also speeds the rate at which dissolved gasses are released from a liquid and enhance the ultrasonic degassing procedure described below. The paragraph above mentioned "nucleation sites." As you all know by now, I am an avid fan of the Discovery Channel program called Mythbusters. In one Mythbusters episode the hosts investigated the science behind the Mentos Fountain. For those of you who have never heard of this, a "Mentos Fountain" is created when Mentos breath mints are added to (preferably) diet cola. The surface of the Mentos mints are rough rather than smooth. This surface roughness provides a multitude of nucleation sites for the gas, CO2, dissolved in the cola to be released from the saturated liquid. The resulting "fountain" is quite spectacular and can reach several tens of feet in height. Just as small bubbles form on the sides of a pot being heated on a stove burner, anything that will help provide nucleation sites in liquids being prepared for ultrasonic activation will speed the degassing process. One way to provide nucleation sites in a cleaning tank is to add the cleaning chemistry early on in the startup process. Agitation is also an effective way to speed the release of dissolved gas from liquids. Those of us who have unwittingly opened a bottle of soda that has been given a good shake by a friend (or sibling) can attest to the effectiveness of agitation in releasing dissolved gas. It turns out that ultrasonic energy is a very effective means of "agitating" liquids. This can be demonstrated by partially immersing a freshly opened bottle of soda in an ultrasonically activated tank of liquid. Ultrasonic energy is effective in degassing liquids by a process I call "rectified diffusion." Since this process is a little involved and difficult to understand for many long time users of ultrasonics not to mention the neophytes, I am providing a short PowerPoint below which describes the process of degassing by rectified diffusion as well as the other methods described above in more detail.
- FJF -